1. Field of the Invention
The present invention relates to a cold cathode electron gun, such as a field emitter array, which can supply a stable electron flow for a long time period by avoiding collisions of electrons against an inner wall of anode.
2. Description of the Prior Art
So far, designing methods concerning hot cathode electron gun have been applied also for designing structures of anode of cold cathode electron guns.
For example, in case of a traveling wave tube, a value of electron current, a radius of electron beam in a slow wave circuit, an inner diameter of helix, a pitch of the helix must be decided on the basis of a product specification such as operating frequency and output power. The radius of electron beam, for example, is set to be about 60% of the inner diameter of helix, taking into consideration manufacturing factors such as a degree of off-axis between the electron lens and the slow wave circuit, and a distortion and curvature of the helix.
In the hot cathode electron gun, the value of electron current is put to be a value of V3/2 multiplied by a perveance which is decided on the basis of shapes of cathode, anode and Wehnelt near the cathode. Here, V is an anode voltage.
Then, on the basis of the decided value of current, the radius of electron beam is calculated by tracking the electrons. Further, shapes of the electrodes are decided to introduce electron beam into the slow wave circuit.
The above-mentioned designing procedures are employed with minute modifications for the cold cathode electron gun.
Some modifications in the designing are necessary, because electrons are emitted with an initial velocity and a divergence angle.
For example, Spindt type cold cathode comprises a cone emitter, and a gate which is provided with a hole which surrounds the pointed end of the cone. Electrons are emitted from the pointed end of the cone by the field-emission under the application of voltage of several tens V to about a hundred V between the emitter and the gate.
Therefore, the electron emitted from the above-mentioned cold cathode has an initial velocity corresponding to the applied voltage, while the initial velocity of the electron emitted from the hot cathode is equivalent merely to thermal energy usually smaller than 1 eV or several eV at most.
Further, in the Spindt type cold cathode, electrons are emitted not only from the pointed end of the cone, but also from micro projections formed on the surface of the cone. Therefore, the emitted electron beam has a divergence angle, because the electric field near the pointed end of the cone is great enough to emit electrons by the field-emission.
The divergence angle indispensable for the electron beam tracing is reported to be 25xc2x0 to 30xc2x0 by P. R. Schwoebel and I. Brodie, in J. Vac. Sci. Technol. B 13 (4) 1391, 1995.
Thus, it is assumed that the emitted electron has an initial velocity of several tens eV and a divergence angle of 25xc2x0 to 30xc2x0, in the electron beam tracing in the electron lens of the cold cathode electron gun and RF circuit such as the slow wave circuit of the traveling wave tube or a resonance cavity of klystron.
An electron gun of which electron flow is stabilized is disclosed for example in JP 10-106430 A (1998). The cold cathode electron gun as shown in FIG. 10, gate electrodes 100 surrounding emitters and cathode electrodes 101 are divided into a plurality of groups. Electrons are extracted from focus electrode 102 at a constant value of current by compensating the surface condition of pointed end of the emitters.
Further, another cold cathode electron gun disclosed in JP 8-106848 A (1996) avoids the collision of electrons against the side wall of focus electrode 13. This cold cathode electron gun as shown in FIG. 11 comprises substrate 14, emitter 15, cathode 11, extracting electrode 12, and focus electrode 13. Insulating film 16b between extracting electrode 12 and focus electrode 13 is over-etched to reduce the width of focus electrode 13 and to avoid the electron collision.
However, the conventional designing procedures for the cold cathode electron gun are not consistent, because merely the design method for the hot cathode is diverted, wherein the initial velocity of the emitted electron is negligibly small.
Further, the emission current decreases after long term operation of the cold cathode electron gun which is designed by the conventional method.
Therefore, an object of the present invention is to stabilize the emission current of the cold cathode electron gun in a long term operation.
A cold cathode electron gun of the present invention comprises: a cold cathode for emitting electrons by field-emission; a gate electrode for controlling the field-emission; a Wehnelt electrode which surrounds the cold cathode and the gate electrode; a first anode for accelerating electrons; and a second anode for constructing an electron lens together with the first anode. In the cold cathode electron gun of the present invention, an inner diameter of the first anode is made greater than a radius of flow of electrons which are emitted in the direction perpendicular to the optical axis of the electron lens.
According to the present invention, the emission current is maintained at the initial value for a long period.
Further, according to the present invention, a product life of the electron gun is extended, because contamination of emitter is reduced.